1. Field of the Invention
This invention relates to a lightweight boom that is carried by a swingable jib of a tower crane adapted for use in the fabrication of annular structures such as concrete hyperbolic cooling towers. The elongated boom has a triangular configuration in a longitudinally transverse cross-section, and a bottom member representing one corner of the triangular configuration is in the form of a cylindrical pipe which has an internal fluid conduit for continuously conveying flowable concrete mix along the length of the boom for introduction into form systems on substantially a continuous basis as may be desired for construction of the tower structure.
2. Description of the Prior Art
For years, it has been common practice to fabricate large annular structures such as concrete hyperbolic cooling towers by use of a rotatable jib mounted on the top of a tower crane, wherein a winch line suspended by the jib successively lowers and raises a concrete mix bucket for conveying concrete from a mix plant or truck to successive locations along an annular form system. The bucket, which usually holds about two cubic yards of concrete, is raised by the winch and the jib is rotated through an arc while the bucket is also moved laterally to a position overlying a location on the forms so that the concrete can be deposited in the next area of the forms to be filled. As can be appreciated, considerable time is wasted during raising and lowering of the bucket, as well as moving the bucket along the length of the jib in such a batch-type operation, especially since as much as 600 cubic yards of concrete must be hoisted to the form system during each working day for the thicker parts of the shell, and since the bucket must be maneuvered about a number of reinforcing bars which project upwardly from the form system along the entire length of the latter.
A significant advance in the art of fabrication of annular concrete structures is taught by the disclosure of U.S. Pat. No. 4,374,790, dated Feb. 22, 1983 and owned by the assignee of the present invention. In U.S. Pat. No. 4,374,790, a method and apparatus is disclosed for continuously pumping flowable concrete mix to an elevated form system by use of a concrete conveying conduit carried by an elongated boom which is suspended by a jib and swingable therewith in an arc to continuously introduce the mix into the entire circumferential length of the form system defining the annular structure. The flow rate of the concrete is selectively controllable for advantageously filling the forms at a uniform, efficient rate without interruptions in the continuity of flow of the concrete mix.
The boom that is illustrated in U.S. Pat. No. 4,374,790 is supported by the jib by means of a primary hoist which is operable to raise and lower the boom, as well as a secondary, chain hoist which enables the boom to be tilted about a horizontal axis to selective vary the longitudinal orientation of the boom with respect to the horizontal. Moreover, the boom of this patent is preferably comprised of detachable sections which enables the length of the boom to be changed in accordance with the diameter of the next annular cooling tower segment to be poured. The boom disclosed in this reference has two parallel, spaced bottom members and a single upper member and presents a triangular configuration in longitudinally transverse cross-section; also, the concrete conveying conduit is carried by the two bottom members in underlying relationship thereto.
The tower crane shown in U.S. Pat. No. 4,374,790 comprises a plurality of stackable units which permits the height of the tower to be increased as necessary as the work progresses. Consequently, the boom of U.S. Pat. No. 4,374,790 can be shifted to a number of various positions so that an outlet spout flexibly connected to the concrete carrying conduit supported by the boom can be conveniently maneuvered along the form structure regardless of the height and diameter of the latter.
However, while the method and apparatus for pumping concrete to elevated form structures according to the disclosure of U.S. Pat. No. 4,374,790 clearly represents a breakthrough in the art, it is nevertheless desirable to provide a means for reducing the weight of the boom carried by the jib so that relatively large cooling towers can be constructed without resorting to the use of exceedingly large and heavy crane structures. In this regard, it has been suggested that the maximum practical length of the boom shown in U.S. Pat. No. 4,374,790 along with the concrete conveying conduit carried therebeneath be limited to approximately 150 feet, due to the relatively heavy structural members comprising the boom as well as the pipe for the concrete (having, for example, an outer diameter of 5.5 inches) and the volume of the concrete that is carried within the pipe at any one time.